1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a uniformly-charged photoconductor (for example, a photoconductor drum) with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet through an intermediate transfer belt, followed by heating and pressurization for fixing at a fixing section, whereby an image is formed on the sheet.
In such an image forming apparatus, as illustrated in FIG. 1A and FIG. 1B for example, conveyance roller section 31 having a pair of rollers 31A and 31B is disposed on the downstream side of fixing section 23 in the sheet conveyance direction, and further, conveyance guide section 32 is disposed on the downstream side of conveyance roller section 31 in the sheet conveyance direction. Conveyance guide section 32 includes first guide member 32A (for example, upper side guide member) disposed on the fixing surface side of a sheet and second guide member 32B (for example, lower side guide member) disposed on the rear surface side. A sheet that has passed through fixing section 23 is output by conveyance roller section 31, and is conveyed along a sheet feeding path defined by conveyance guide section 32.
In addition, in an image forming apparatus that can form images on both sides of a sheet, switch-back conveyance section 162 for conveying a sheet to a rear surface printing conveyance section is provided continuously with second guide member 32B, and the sheet feeding paths are switched by feeding-path-switching section 164. FIG. 1A illustrates a sheet feeding path for the case where a sheet is ejected out of the apparatus, and FIG. 1B illustrates a sheet feeding path for the case where a duplex image formation is performed.
Typically, conveyance roller section 31 is disposed such that a sheet being output is substantially parallel to second guide member 32B regardless of one-side image formation or duplex image formation. Here, angle θ of tangential direction L of a conveyance nip formed by a pair of rollers 31A and 31B to second guide member 32B is referred to as “nip angle” (see FIGS. 2A and 2B). That is, nip angle θ is an angle between perpendicular P of second guide member 32B and line M that connects the axial centers of rollers 31A and 31B. Typically, nip angle θ of conveyance roller section 31 is set to 0° as illustrated in FIG. 2B.
Incidentally, in fixing section 23, the temperature of a fixing side member is higher than the temperature of a back side supporting member, and therefore a protruding curl is easily formed on the fixing surface side (on first guide member 32A side) of a sheet after the fixation. In this case, there is a risk that the sheet image formation surface makes contact with first guide member 32A. When a sheet that has passed through fixing section 23 makes contact with the conveyance surface of first guide member 32A, the image formation surface is chipped by burrs of a resin rib and a sheet metal, and consequently scratch may possibly be left in the image. In particular, scratch is easily caused in the field of production print where image formation is continuously performed with high speed.
Conventionally, scratch has been avoided by disposing a conveyance roller that rotates to follow the conveyance of sheets and protrudes into the sheet feeding path relative to the conveyance surface in the first guide member and the second guide member (for example, Japanese Patent Application Laid-Open Nos. 2003-270995 and 2008-179442). In addition, Japanese Patent Application Laid-Open No. 10-319647 discloses a technique in which, in the case where a sheet is output to a rear surface printing conveyance section after fixation to perform duplex image formation, the nip angle of the conveyance roller section is changed to moderate the angle of the sheet entering the feeding-path-switching section (switching gate).
In the following, a surface on which an image is formed in one-side image formation or a surface on which an image is firstly formed in duplex image formation is referred to as “first surface,” and the rear surface of the first surface is referred to as “second surface.”
As disclosed in Japanese Patent Application Laid-Open Nos. 2003-270995 and 2008-179442, it is effective to dispose a conveyance roller in the sheet feeding path to deal with the scratch problem. However, the image forming apparatus may not have the space for disposing the conveyance roller. In addition, even if the space for disposing the conveyance roller is secured, jam and corner folding of sheets are easily caused since the angle of the sheet entering the conveyance roller is great, and therefore installation of the conveyance roller may not always be preferable.
In addition, the technique disclosed in Japanese Patent Application Laid-Open No. 10-319647 is intended to cause sheets to smoothly enter the feeding-path-switching section, and is not useful for preventing scratch. Specifically, since the nip angle is changed to the direction in which the impact of the contact between the first surface of the sheet and the first guide member is increased, it can be said that scratch is caused more easily.